[dfs_v2] fix dfs_pcache compilation error if no mmu.h

[koendv]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

When using dfs_v2 without page cache, dfs_pcache.c gives a compilation error if there is no mmu.h.

CC build/kernel/components/dfs/dfs_v2/src/dfs_pcache.o
src/rt-thread/components/dfs/dfs_v2/src/dfs_pcache.c:19:10: fatal error: mm_page.h: No such file or directory
   19 | #include <mm_page.h>

你的解决方案是什么 (what is your solution)

In dfs_pcache.c, put the #ifdef RT_USING_PAGECACHE before the #include. Compiles. Runs.

 \ | /                                                  
- RT -     Thread Operating System                      
 / | \     5.2.0 build Nov 13 2024 07:47:59             
 2006 - 2024 Copyright by RT-Thread team
msh />ls /dev
Directory /dev:
pin                 <CHR>                    
uart1               <CHR>                    
msh />

请提供验证的bsp和config (provide the config and bsp)

• BSP: at32f435-start

• .config:
  config.txt
  CONFIG_RT_USING_DFS=y
  CONFIG_RT_USING_DFS_V2=y
  CONFIG_RT_MAIN_THREAD_STACK_SIZE=4096

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用formatting 等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/workflows/bsp_buildings.yml 详细请参考链接BSP自查

[BernardXiong]

Thank you for your valuable contribution. I would appreciate it if you could share more details regarding the resource consumption of DFS V2, specifically concerning ROM and RAM usage. Your insights are greatly appreciated.

[koendv]

Thank you. You're right, for many processors dfs_v2 is too big.